"[He told stories] like I was strapping one leg behind my back and hopping on that incessantly. Just when they thought I was ready to go to set, I'd strap the other leg behind my back and do that. It's all lies. It's an interesting story," said Fassbender, who plays legendary psychotherapist Carl Jung in the thriller.

"He told me, 'Did you get that email I sent you?' and I said no. It's hard to contact Viggo - even the studios never know when he's going to show up.

"He sent me an email since The Hollywood Reporter or something asked what it was like to work with me, so he sent that e-mail about the leg story. I guess he got his chance last week [on this show]!"

Letterman then asked Fassbender if he had any rumours that he would like to start about Mortensen.

"He sits in a corner naked eating a banana!" Fassbender joked.

A Dangerous Method, which also stars Keira Knightley, is released in the UK on February 10.